In this specification the term “light” will be used in the sense that it is used in optical systems to mean not just visible light, but also electromagnetic radiation having a wavelength outside that of the visible range.
It is well-known to control the bias setting of a Mach-Zehnder (MZ) modulator by adding a “tone” to the modulator bias having a low frequency that can be detected using photodetectors and electronics operating at speeds much lower than the modulation, allowing for low noise and low cost. This low frequency tone is input to the modulator and monitored by a photodetector (PD) at an output of the modulator as an average and the bias voltage of the modulator is adjusted until a minimum optical output is found. The tone applied to the modulator increases and decreases the bias voltage such that a minimum optical output can be found when the output measured by the PD rises with either an increase or a decrease in bias voltage. This process is termed “dithering” of the bias voltage.
Alternatives to the method of dithering the bias voltage are known as set out, for example in U.S. Pat. No. 6,700,907, in which a controller executes interleaved DC bias and gain control routines using monitored values of the monitored photocurrent output signal of a MZ laser modulator to derive a gain setting for the modulation drive signal and a bias level setting for the DC bias level.
There is presently a market need to reduce the power consumption of devices such as indium phosphide (InP) modulators and so there is a desire to use lower drive voltages for those modulators. The signal from a modulator driven at a lower amplitude may be amplified by a semiconductor optical amplifier (SOA) in order to reach the required line transmission power. Overall the combination of low power driven MZs and an SOA results in a device consuming less power and radiating less heat. Lower drive voltages result in a lower amplitude of signal at the output of the MZ and so, when a dither tone is used to control the bias point, the amplitude of the tone can become significant relative to the output signal and cause transmission problems.
Broadly, the prior art uses a control system to place a monitor PD operating point at a minimum for phase modulation (or maximum for a complementary output). The control system could use hill-climbing or dithering to find the maximum/minimum. In any case, the system requires some movement off the ideal operating point to detect the maximum/minimum.